ANSI-C was not designed with embedded controllers in mind. The listed keywords in the following listing do not conform to ANSI standards. However, they enable you to achieve good results from code used for embedded applications.
__alignof__ __va_sizeof__ __interrupt (alias interrupt) __asm (aliases _asm and asm)
You can use the __interrupt keyword to mark functions as interrupt functions, and to link the function to a specified interrupt vector number (not supported by all backends).